No, it could also mean he simply let go of his wife, whether dreaming or not.

I think so too. Regardless of the idea that Cobb let go of his wife and reunited with his children or not, that ring symbolizes the intent to the viewer with much more directness.

Using the ring as an indicator allows someone to nit pick that he may be wearing the ring in that scene and that something happened between the airport and the car ride home.

Using the fact that he didn't care to verify solidifies the real result... he's given up fighting.

The two aren't necessarily incompatible though are they?

Whether Cobb cares or not he's still somewhere.

Or you can look at it that there are two endings depending on what you're looking for - certainty or something other.